Desalination and purification system

ABSTRACT

A liquid electrolyte can be desalinated and purified using a system that includes a first electrode and a configuration selected from (a) a second electrode and at least one distinct ion-selective boundary and (b) a second electrode that also serves as the ion-selective boundary. The ion-selective boundary is contained in the liquid conduit adjacent to a porous medium that defines pore channels filled with the liquid and that have a surface charge, and the charge of the ion-selective boundary and the surface charge of the pore channels share the same sign. A liquid including at least one charged species flows through the pore channels, forming a thin diffuse electrochemical double layer at an interface of the liquid and the charged surface of the pore channels. A voltage differential is applied between the electrodes across the porous medium to draw ions in the liquid to the electrodes to produce brine at the electrodes and to create a shock in the dissolved-ion concentration in the bulk volume of the liquid within the pore channels, wherein a depleted zone with a substantially reduced concentration of dissolved ions forms in the liquid bulk volume between the shock and the ion-selective boundary.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.13/165,042, filed 21 Jun. 2011. This application also claims the benefitof U.S. Provisional Application No. 61/356,769, filed 21 Jun. 2010. Theentire content of both of these applications are incorporated herein byreference.

BACKGROUND

In this century, the shortage of fresh water is expected to surpass theshortage of energy as a global concern for humanity, and these twochallenges are inexorably linked. Fresh water is one of the mostfundamental needs of humans and other organisms. Each human needs toconsume a minimum of about two liters per day, in addition to greaterfresh-water demands from farming as well as from industrial processes.Meanwhile, techniques for transporting fresh water or for producingfresh water via purification and desalination of seawater, brackishwater, waste water, contaminated water, etc. tend to be highly demandingof increasing scarce supplies of affordable energy.

The hazards posed by insufficient water supplies are particularly acute.A shortage of fresh water may lead to famine, disease, death, forcedmass migration, cross-region conflict/war (from Darfur to the Americansouthwest), and collapsed ecosystems. In spite of the criticality of theneed for fresh water and the profound consequences of shortages,supplies of fresh water are particularly constrained. 97.5% of the wateron Earth is salty, and about 70% of the remainder is locked up as ice(mostly in ice caps and glaciers), leaving only 0.75% of all water onEarth as available fresh water.

Moreover, that 0.75% of available fresh water is not evenly distributed.For example, heavily populated developing countries, such as India andChina, have many regions that are subject to scarce supplies. Furtherstill, the supply of fresh water is often seasonally inconsistent.Typically confined to regional drainage basins, water is heavy and itstransport is expensive and energy-intensive.

Meanwhile, demands for fresh water are tightening across the globe.Reservoirs are drying up; aquifers are falling; rivers are drying; andglaciers and ice caps are retracting. Rising populations increasedemand, as do shifts in farming and increased industrialization. Climatechange poses even more threats in many regions. Consequently, the numberof people facing water shortages is increasing.

Even when fresh water is available, billions of people live withunacceptable levels of contamination. There is a growing need for waterpurification systems that can remove not only common ions, but alsovarious dangerous trace impurities such as arsenic, copper, radioactiveparticles, fertilizers, bacteria, viruses, etc. many of which aredifficult to eliminate efficiently with traditional filters andmembranes. Cheap, low-power, portable systems could have a major impacton public health, if they could be easily deployed to remote orunder-developed locations with poor or nonexistent water distributioninfrastructure.

Massive amounts of energy are typically needed to produce fresh waterfrom seawater (or to a lesser degree, from brackish water), especiallyfor remote locations. Reverse osmosis (RO) is currently the leadingdesalination technology, but it is energy intensive and still relativelyinefficient due to the large pressures required to drive water throughsemi-permeable membranes and their tendency for fouling. In large-scaleplants, the energy/volume required can be as low as 4 kWh/m³ at 30%recovery, compared to the theoretical minimum around 1 kWh/m³, althoughsmaller-scale RO systems (e.g., aboard ships) can have much worseefficiency, by an order of magnitude.

Rather than extracting pure water, electrochemical methods, such aselectrodialysis (ED) and capacitive desalination (CD), extract justenough salt to achieve potable water (<10 mM). Current large-scaleelectrochemical desalination systems are less efficient than RO plantsat desalinating seawater (e.g., 7 kWh/m³ is the state of the art in ED),but become more efficient for brackish water (e.g., CD can achieve 0.6kWh/m³). These electrochemical methods also offer advantages forefficient high-recovery purification of partially or completelydesalinated water, by expending energy mainly to remove just theundesirable particles, rather than most of the water molecules, from thesolution. Existing ED and CD methods, however, do not reach the samelevel of water purity as RO, since some undesirable particles can flowpast the electrodes or membranes.

SUMMARY

Described herein are methods and apparatus for desalination (saltremoval) and liquid purification (particulate removal) using macroscopicporous media and membranes, exploiting the formation of sharp gradientsin salt concentration, which we call “desalination shocks”, driven bysurface conduction and electro-osmotic flow. Various embodiments of theapparatus and method may include some or all of the elements, featuresand steps described below.

In the apparatus, a conduit is provided for liquid flow therethrough,and at least two electrodes are configured to drive ionic current inliquid flowing from an inlet port to a desalinated/purified liquidoutlet port in the conduit. At least one ion-selective boundary (e.g.,ion-exchange membrane) is configured to conduct the ionic current andselectively transmit or remove counter-ions while blocking co-ions fromthe liquid, and at least one porous medium is adjacent to theion-selective boundary (i.e., the porous medium is not necessarily incontact with the ion-selective boundary; though if separated, theseparation distance is very small, of the order of the screening length,e.g., of the order of 2-100 nm in aqueous solutions) and on an oppositeside of the ion-selective boundary from the second electrode in theconduit. The porous medium has a surface charge with a sign that is thesame as the sign of the co-ions to enable conduction of an ionic surfacecurrent (in the double layers) carried by the counter-ions andconsequent production of a region of desalinated/purified liquid,wherein the desalinated/purified liquid outlet port is positioned toextract the desalinated/purified liquid from the porous medium. Themethod and apparatus involve the formation of a sharp salt concentrationgradient (i.e., a “desalination shock”) in a region of the porous mediumnear the membrane. The desalination shock enables membrane-lessseparation (i.e., where ions can be separated and removed in the porousmedium without needing a physical barrier or membrane at the separationlocation). In this shock region, the rate of change in saltconcentration as a function of distance from the membrane issubstantially greater than it is elsewhere across the continuous porechannels in the porous medium. A depleted region of lower salinity(e.g., fresh water) is thereby produced in the bulk liquid between theshock region and the membrane. In addition to classical bulk diffusion,ion transport from the liquid is enhanced by surface conduction withinthe screening cloud (or double layers) within the pores, and the removalof, e.g., fresh water from the depleted region can be driven byelectro-osmotic and/or pressure-driven flow.

In some embodiments, one of the electrodes can serve as theion-selective boundary. For example, a porous metal electrode can storecounterions capacitively in its double layers, while rejecting co-ions.An electrode undergoing electrodeposition of metal ions (or otherelectrochemical adsorption/deposition processes) can achieve the sameresult. In a particular example, the ion-selective boundary is a coppercathode that removes copper ions from a copper chloride aqueous solutionin a packed bed of silica microspheres by electrodeposition. Thisremoval of the copper ions triggers the same “desalination shock”phenomenon in the porous medium leading to over-limiting current anddesalinating the copper chloride solution.

Multiple assemblies can be stacked in parallel to boost the flow rate.As in traditional electrodialysis, brine can be produced in theelectrode compartments by redox reactions and removed by pressure-drivenflow, though the porous medium in this apparatus provides a new methodof fresh water recovery and particle filtering.

These methods and systems can be applied in low-cost, low-voltage,macroscopic systems to produce useful flow rates for both small-scaleand large-scale applications and can be used with a variety of watersources, including seawater, brackish water, sewage, industrialwastewater, contaminated drinking water, oil-well wastewater andagricultural wastewater, or with other liquids. In one example, theapparatus is powered by a battery or by solar panels coupled with thesystem. Suitable applications include small-scale uses in remote regionswith limited access to fresh water and energy and/or in the military,wherein the apparatus can be transported by individual soldiers orgroups of soldiers, or in a vehicle. In other embodiments, the systemcan be coupled with the electrical grid for large-scale fresh-waterproduction. The system can be used for a variety of purposes, includingdesalination and purification of sea water or brackish water, as well asionic liquids or electrolytes that are not water-based (such asalcohol-based electrolytes, organic electrolytes, surfactant-stabilizedcolloids or micelles in non-polar solvents, etc.), or cleaning porousmaterials or soils by flowing a liquid therethrough and extracting ionsand counter-ions from the porous material through the system.

The methods and systems can also provide ultra-filtration with reducedmembrane filtering; and fresh water produced with these methods andsystems can be free of negatively charged impurities (such as most dirtand viruses), allowing only positively-charged particles that fitthrough the pores (e.g., having a diameter less than 100 nm) to passthrough the shock region. Accordingly, the porous medium and shockregion can also protect the membrane surface from fouling by preventingparticles from reaching the membrane. The porous medium can also be ofan inexpensive composition and, in particular embodiments, is easy toclean or replace (whereas, the membrane, if fouled, typically iscomparatively difficult to clean and expensive to replace).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic elements of a desalination and purificationsystem, including a cationic porous medium (CPM) with negatively chargedpores in contact with a cation exchange membrane (CEM).

FIG. 2 shows another embodiment of the system of FIG. 1, where thecationic porous medium is a packed bed of micron-sized negativelycharged beads (e.g., silica or latex) in a liquid-filled tube or column.

FIGS. 3 and 4 are illustrations intended to explain the basic physics ofdesalination shock formation in a charged pore filled with a liquidelectrolyte.

FIG. 5 shows another embodiment of the desalination and purificationsystem, including an anionic porous medium (APM) with positively chargedwalls in contact with an anion exchange membrane (AEM) and with adesalination shock formed in the anionic porous medium.

FIG. 6 shows how the components of FIGS. 1 and 5 can be combined in atwo-electrode device to produce fresh liquid in two locations behind tworesulting shocks.

FIGS. 7 and 8 show periodic repetitions of the elements above inparallel stacks, to achieve larger flow rates. FIG. 7 shows analternating stack of combined elements from FIG. 5. FIG. 8 shows analternating stack of the components in FIG. 1.

FIG. 9 shows a periodic repetition of structures comprising APM and CPMsandwiched between AEM and CEM.

FIG. 10 shows a sandwich structure of FIG. 9 with a plot of thetransient salt concentration profile in one APM/CPM section.

FIG. 11 shows a stack of alternating CPM/CEM layers leading tomembrane-less separation of salt in the CPM layers.

FIG. 12 shows a CPM sandwiched between two CEM layers with a plot of thesteady salt concentration profile in the CPM.

FIG. 13 shows a cylindrical configuration of the system shown in FIG.11.

FIG. 14 shows a cylindrical configuration of the system shown in FIG. 9.

FIG. 15 shows a system with a heterogeneous porous medium, wherein theshock region is pinned at the interface between a finer-pore zone and alarger-pore zone.

FIG. 16 shows a system with a heterogeneous porous medium that includesa lower-surface-charge zone and a higher-surface-charge zone.

FIG. 17 shows a first method of fresh water recovery by allowing freshliquid to escape via a gap in the sidewall of the conduit near theCPM/CEM interface and over the depleted zone in the cationic porousmedium.

FIG. 18 shows a complete shock membrane apparatus, exploiting thedesalination method of FIG. 1 and the water recovery method of FIG. 17.A weak pressure-driven flow is imposed (e.g., by gravity) to removebrine from the anode and cathode compartments and augment theelectro-osmotic flow through the cationic porous medium driving freshliquid out near the cation exchange membrane.

FIG. 19 shows a second method and apparatus of water recovery, usingtransverse-pressure-driven flow in the cationic porous medium. The shocktraverses some or all of the cationic porous medium while bending in theflow, and the depletion zone extends to the side of the cationic porousmedium, where fresh liquid is removed.

FIG. 20 shows another method and apparatus for water recovery, againusing pressure-driven flow—here with both anionic-porous-medium andcationic-porous-medium elements in contact. The two shocks emanatingfrom the two membranes collide prior to the side exit to allow easyremoval of fresh liquid.

FIG. 21 shows an embodiment of the apparatus in FIG. 17, including asandwich of the different elements, cut to have the same cross sectionand glued together.

FIG. 22 shows a third method of water recovery, wherein onemembrane-and-electrode assembly, the CEM/cathode (or AEM/anode), isplaced on the side of the porous cationic (or anionic) porous medium,transverse to the other electrode, the anode (or cathode). The geometryallows electro-osmotic flow transverse to the membrane to contributedirectly to water recovery, aided by possible pressure-driven flow alongthe same axis.

FIG. 23 shows another embodiment of the apparatus of FIG. 22, where theCPM/CEM/cathode assembly has the form of a coaxial tube. Fresh wateremerges along the axis from the opposite end of thecationic-porous-medium core, while brine is produced in the outerannulus of the tube from the cathode, as well as on the front end of thetube, from the anode.

FIG. 24 shows the liquid flows for the system illustrated in FIG. 14.

FIGS. 25 and 26 illustrate the trade-off between flow rate anddesalination factor via the slow flow rate in FIG. 25, which allows theshock to extend past the outlet into the brine channel, in comparisonwith the high flow rate in FIG. 26, which causes the shock to leave theporous medium in the fresh water outlet, along with salty or unpurifiedinput fluid.

FIGS. 27 and 28 illustrate the trade-off between water recovery(Q_(fresh)/Q_(in)) and desalination factor by controlling the outletflow rate ratio (Q_(fresh)/Q_(brine)), wherein FIG. 27 shows excellentdesalination at low water recovery (and at high energy cost/volume),while the system in FIG. 28 produces much more salt in the fresh wateroutlet with high water recovery.

FIG. 29 illustrates principles of water purification by desalinationshocks. Large particles are filtered by size before entering the porousmedium. Sufficiently small co-ionic particles end up rejected by chargeto the brine outlet stream. Only sufficiently small counterion-ionicparticles can make it to the fresh outlet stream.

FIG. 30 shows another embodiment of the element in FIG. 1, including thepathways for fluid flow, as in the experimental results reported below.

FIGS. 31 and 32 show simulated data for the bulk conductivity profile ina nano/micro pore. FIG. 31 shows the conductivity profile for varyingvalues of applied voltage. FIG. 32 shows the conductivity profile forseveral Pe values.

FIG. 33 shows simulated V-I curves for varying surface charge densitiesand Peclet numbers for the system represented in FIGS. 31 and 32.

FIG. 34 shows energy/volume calculations for the system represented inFIGS. 31 and 32 for desalination of seawater (0.5 M).

FIG. 35 is a sectional view of a shock-membrane prototype for CuSO₄“desalination.”

FIG. 36 is a magnified view of a section from FIG. 35.

FIG. 37 is a photographic image of the shock-membrane prototype inoperation.

FIG. 38 plots the first experimental results for the shock-membraneprototype of FIG. 37 for continuous CuSO₄ desalination.

FIG. 39 plots the impedance spectra for the initial solution used in theexperiment for which the results are plotted in FIG. 38.

FIG. 40 plots the impedance spectra for the extracted solution used inthe experiment for which the results are plotted in FIG. 38.

In the accompanying drawings, like reference characters refer to thesame or similar parts throughout the different views. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating particular principles, discussed below.

DETAILED DESCRIPTION

The foregoing and other features and advantages of various aspects ofthe invention(s) will be apparent from the following, more-particulardescription of various concepts and specific embodiments within thebroader bounds of the invention(s). Various aspects of the subjectmatter introduced above and discussed in greater detail below may beimplemented in any of numerous ways, as the subject matter is notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes.

Unless otherwise defined, used or characterized herein, terms that areused herein (including technical and scientific terms) are to beinterpreted as having a meaning that is consistent with their acceptedmeaning in the context of the relevant art and are not to be interpretedin an idealized or overly formal sense unless expressly so definedherein. For example, if a particular composition is referenced, thecomposition may be substantially, though not perfectly pure, aspractical and imperfect realities may apply; e.g., the potentialpresence of at least trace impurities (e.g., at less than 1 or 2% byweight or volume) can be understood as being within the scope of thedescription; likewise, if a particular shape is referenced, the shape isintended to include imperfect variations from ideal shapes, e.g., due tomachining tolerances.

Although the terms, first, second, third, etc., may be used herein todescribe various elements, these elements are not to be limited by theseterms. These terms are simply used to distinguish one element fromanother. Thus, a first element, discussed below, could be termed asecond element without departing from the teachings of the exemplaryembodiments.

Spatially relative terms, such as “above,” “upper,” “beneath,” “below,”“lower,” and the like, may be used herein for ease of description todescribe the relationship of one element to another element, asillustrated in the figures. It will be understood that the spatiallyrelative terms, as well as the illustrated configurations, are intendedto encompass different orientations of the apparatus in use or operationin addition to the orientations described herein and depicted in thefigures. For example, if the apparatus in the figures is turned over,elements described as “below” or “beneath” other elements or featureswould then be oriented “above” the other elements or features. Thus, theexemplary term, “above,” may encompass both an orientation of above andbelow. The apparatus may be otherwise oriented (e.g., rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein interpreted accordingly.

Further still, in this disclosure, when an element is referred to asbeing “on,” “connected to” or “coupled to” another element, it may bedirectly on, connected or coupled to the other element or interveningelements may be present unless otherwise specified.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting of exemplary embodiments.As used herein, singular forms, such as “a” and “an,” are intended toinclude the plural forms as well, unless the context indicatesotherwise. Additionally, the terms, “includes,” “including,” “comprises”and “comprising,” specify the presence of the stated elements or stepsbut do not preclude the presence or addition of one or more otherelements or steps.

In the following examples, we assume without loss of generality that theliquid is water containing dissolved salts and charged impurities to beremoved by the apparatus and methods described herein. It will beunderstood that the same apparatus and methods can be applied by thoseskilled in the art to other liquids containing dissolved ions and/orcharged impurity particles. Some examples are given below, afterembodiments are specified, in detail, for the applications dealing withdesalination and purification of aqueous solutions.

A desalination and purification system 10 is shown in FIG. 1. A cationicporous medium (CPM) 12 with negatively charged pore channels 14 is incontact with a cation exchange membrane (CEM) 16. A liquid includingco-ions and oppositely charged counter-ions, charged impurities and/orcharged droplets flows left-to-right as shown through the cationicporous medium 12. Direct electric current is passed from the cationicporous medium 12 through the cation exchange membrane 16, and adesalination shock forms at the CPM/CEM interface and propagates intothe cationic porous medium 12, leaving behind a depleted region of freshwater (the term, “fresh water,” as used herein, can represent potablewater having less than approximately 10 mM of dissolved salts).Particles suspended in the input stream are also rejected by size orcharge at the entrance to the cationic porous medium (at the left sideof the cationic porous medium 12 in FIG. 1) and are further rejected bythe shock within the cationic porous medium 12. The direction of flowfor anions and cations in the system are shown with respective arrows.Though a gap for liquid flow is shown here between the anode 19 and thecationic porous medium 12, the anode 19 and cationic porous medium 12can be in flush contact in other embodiments, and the source liquid canbe directly injected into the porous medium 12.

The porous medium 12 has a rigid structure and has ideally a highsurface charge. In one embodiment, the cationic porous medium 12 is aporous glass frit with approximately 1-micron pores, and the cationexchange membrane 16 is formed of a sulfonated-tetrafluroethyele-basedfluoropolymer-copolymer (commercially available as a NAFION membranefrom E. I. du Pont de Nemours and Company), which is assembled togetherwith a porous cathode 18. Alternatively, the cationic porous medium 12can take many other naturally occurring or artificially fabricatedforms, such as the following:

-   -   electrochemically prepared porous materials, such as anodic        aluminum oxide with parallel nanopores;    -   fused or packed beds of silica beads, latex spheres, or other        colloid particles (see embodiment, discussed below);    -   zeolite materials;    -   other types of porous glass or ceramic frits;    -   porous polymer materials,    -   functionalized polymers with large negative surface charge,    -   cross linked polymers; or    -   porous metals or semiconductors with oxide coatings.        The cationic porous medium 12 can also be made from any of the        following:    -   polydimethylsiloxane (PDMS),    -   polymethylmethacrylate (PMMA),    -   other elastomeric materials,    -   etched or milled glass,    -   silicon or other semiconductors, or    -   other solid materials with micro/nano-fabricated artificial pore        networks extending therethrough.

The porous material may also contain ion-exchange resins or nanoporousmaterials to enhance counterion conductivity to the counterion-selectiveboundary. This will promote desalination shocks leading to strong saltdepletion in the larger pores if the conduction paths for counterionshave few interruptions. If, however, as in packed beds ofion-exchangers, the conduction paths terminate and produce transientenrichment and depletion regions at the pore scale, then mixing due toconcentration polarization and nonlinear electro-osmotic flows canprevent the formation of desalination shocks, or cause them to widen,thereby lowering their salt separation efficiency. For this reason,particular embodiments include porous materials that have porethicknesses that mostly fall into an optimal range of negligibledouble-layer overlap and suppressed convection within the pores (e.g.,100 nm to 10 microns in aqueous solutions). The microstructure can alsobe anisotropic to optimize surface conduction to the membrane, whileallowing for transverse flow to extract the desalinated fluid, asdescribed below.

In the embodiment of FIG. 2, the cationic porous medium 12 is a packedbed of microspheres formed, e.g., of silica or latex. The microspherescan be loaded by flow into a tube 58 and condensed toward the end bycentrifuge. This method also allows for easy porosity grading or othercontrol of the spatial variation in microstructure of the cationicporous medium 12, by varying the loading strategy or type of particles.For example, finer particles can be employed (thereby reducing porevolume) in regions of the system where restricted fluid flow is desired.Accordingly, the packed bed of particles can have non-uniform porosity,surface charge, or microstructure on a scale at least an order ofmagnitude greater than average particle diameter to promote increasedliquid flow through particular regions of the system. In otherembodiments, the surface charge or microstructure can be substantiallyvaried in different regions to control the spatial distribution of fluidflow through the system. Pressure buildup, however, may disrupt thepacking and interfere with stable shock formation.

Many of the same types of materials can also be used in an anionicporous medium 13 (see FIG. 5), with positive surface charge, such aspositively charged polymeric porous materials, positively chargedself-assembled monolayers or thin films on porous substrates, orartificially prepared surfaces by polymeric layer-by-layer deposition,starting from any of the cationic porous media listed above. With any ofthese cases of cationic or anionic porous media, the surface charge,surface ionic mobility, and/or electro-osmotic slip mobility can beenhanced by surface treatments, films, coatings, or self-assembledmolecular layers.

To illustrate the principles behind the formation of the desalinationshock, a channel for electrolyte liquid flow through the pore channels14 in the cationic porous medium 12 is shown in FIGS. 3 and 4. As shownin FIG. 3, the pore walls 20 of the cationic porous medium 12 have anegative charge and attract excess positive ions from the liquid to formdouble layers 24 at the interfaces of the pore walls 20 and the liquid22. Typically, the double layers 24 are thin compared to the channelthickness. The pore channels have a sufficient diameter, h_(p) (e.g., atleast 50 nm in water, or more generally, greater than the Debye length),to prevent overlap of double layers on opposite sides of the porechannel. The liquid volume 26 bounded by the double layers 24 is termedthe “bulk liquid.” More precisely, when the excess salt in the doublelayers 24, relative to the quasi-neutral solution, is subtracted, whatremains is the effective volume of bulk electrolyte 26 filling thepores. As shown, the pore size, h_(p), is greater than the Debyescreening length, λ_(D); and λ_(D) is below 1 nm in seawater and can beas large as 100 nm in deionized water.

The pore channel 14 can be conceptually divided into three regions, asshown in FIG. 4. The liquid in the bulk volume 26 has a high chargecontent, or ionic conductivity, on the left side (as shown) where theinitial liquid is introduced; as a result, electric current flowsprimarily through the bulk liquid here. In the center is a “shockregion” 28 in which current flows shift from being primarily in the bulkliquid volume 26 (on the left side) to being primarily in the doublelayer 24 (to the right). The bulk liquid volume 26′ in the region 30 tothe right of the shock region 28 is depleted (i.e., has a very lowcontent of charged ions, particles or droplets); consequently, theelectrical resistance in the bulk liquid volume 26′ in the depletedregion 30 is lower than the electrical resistance along the double-layerinterface 24. Accordingly, this depleted bulk liquid volume 26′ can beregarded as being desalinated and/or purified compared with the initialliquid fed into the system 10. The fundamental mechanism for theformation of the depleted region 30 is surface conduction through thedouble layers 24, which becomes increasingly important compared toclassical diffusion in the bulk liquid 26/26′, as the salt concentrationis reduced by the ion-selective surface (of the membrane 16 or electrode18). The basic physics of desalination shocks are described in A. Maniand M. Z. Bazant, “Desalination Shocks in Microstructures,” which wasincluded in the parent provisional application (U.S. Ser. No.61/356,769).

An alternative embodiment of the system is shown in FIG. 5, wherein thepore walls 20 of an anionic porous medium 13 have a positive charge,thereby attracting a thin layer of negatively charged ions from theliquid 22 at the interface of the pore walls 20 and the liquid 22. Inthis embodiment, the membrane is an anionic exchange membrane 17 tofacilitate the transport of negatively charged ions therethrough. Duringthe passage of direct current from the anion exchange membrane throughthe anionic porous medium, a desalination shock forms in the porousmedium, again leaving behind a depleted zone of desalinated and purified(“fresh”) liquid (e.g., fresh water).

In additional embodiments, as shown in FIG. 6, a combination of cationicand anionic shock elements, including both cationic and anionic exchangemembranes 16 and 17 and adjacent porous media 12 and 13 can be provided.These elements can be periodically repeated in a stack configuration.Though gaps are shown between the cationic porous medium 12 and theanionic porous medium 13 in FIGS. 6 and 7; these layers can be in directcontact in other embodiments. Stacked configurations are shown in FIGS.7 and 8. The stacked system of FIG. 7 includes both anionic and cationicmedia and membranes, while the stacked system of FIG. 8 includes onlycationic media and membranes, along with anodes 19 and cathode 18.

Another stack configuration is shown in FIG. 9, which includes a serialrepetition of sandwiches of an integrally coupled anionic porous medium13 and a cationic porous medium 12 between an anion exchange membrane 17and a cation exchange membrane 16, wherein negative charges flow to theleft toward the anode 19 and positive charges flow to the right towardthe cathode 18. This structure is, in some ways, similar to a standardelectrodialysis structure, except that the dialysate channels are filledwith anionic porous medium 13/cationic porous medium 12 sandwichstructures. In contrast to electrodialysis, these porous media are usedto produce desalination shocks that drive localized “membrane-less”desalination and purification processes within the porous media.

One anionic porous medium 13/cationic porous medium 12 sandwichstructure from FIG. 9 is shown in FIG. 10 along with the transient saltconcentration profile across the structure at constant current. Thefollowing four regions are shown in the corresponding plot of saltconcentration (c₀) versus horizontal coordinate: region 63 (initialdepletion by diffusion), region 65 (shock propagation), region 67 (shockcollision), and region 69 (exponential relaxation to completedesalination).

A stack of alternating cationic porous media 12/cation exchangemembranes 16 are shown in FIG. 11, wherein anions migrate across thelayers toward the anode 19, while cations migrate toward the cathode 18.This embodiment exemplifies “membrane-less separation” of salt in thecationic porous media 12 layers, as shown in FIG. 12. In FIG. 12, onelayer of porous medium 12 from FIG. 11 is shown sandwiched between apair of adjoining cation exchange membranes 16, and the saltconcentration across this structure is plotted above. As shown in theplot, a depletion (desalinated) zone 30 of relatively pure water isformed on the right side of the cationic porous medium 12, while a brinezone 35 with a high salt concentration is formed on the left side of thecationic porous medium 12. The initial salt concentration (c₀) of theliquid is shown with the hashed line in the plot. This embodimentclearly shows the difference between desalination shock propagation andstandard electrodialysis because it uses only one type of membrane,augmented by a porous material of the same surface charge. Thedesalination shock phenomenon leads to depletion of both cations andanions in the same location. In contrast, electrodialysis involves twodifferent types of membranes, a cation-exchange membrane of negativeinternal charge to extract the cations and anion-exchange membrane ofpositive internal charge to extract the anions.

The various designs shown herein can be fabricated with the samesequence of structures along the current path but in alternativegeometries. For example, the structure of FIG. 11 is reconfigured inFIG. 13 into a cylindrical configuration, while the structure of FIG. 9is likewise reconfigured into a cylindrical configuration in FIG. 14.

In the various systems shown herein, heterogeneous porous materials(with spatially varying properties) can be used to control the locationof the shock. For example, the shock 28 can be pinned at the interfacebetween two regions of cationic porous media 12 with differently sizedparticles. As shown in FIG. 15, the top region 81 of the cationic porousmedium can have larger particles and pores, while the bottom region 83of the cationic porous medium can have finer particles and pores, withthe shock 28 pinned at the interface of regions 81 and 83. In theembodiment of FIG. 16, the shock 28 is pinned at the interface of alower-surface-charge region 85 and a higher-surface-charge region 87. Inthis way, the shock 28 can be “aimed” at splitting of the brine andfresh water outgoing streams (discussed, below), such that thedesalinated liquid covers the fresh-water outlet, helping to optimizeoperation of the system.

Recovery of desalinated water 32 is shown in FIG. 17. The recoverymethod exploits the pressure build-up in front of the membrane 16 due toelectro-osmotic flow to eject fresh water from the depletion zone 30,behind the shock region 28 and in front of the membrane 16. The flow 34is driven by electro-osmosis, wherein the liquid flow 34 through thesystem 10 coincides with the electric current flow 36 in the system 10,though the liquid flow 34 can be augmented by a pressure-driven flow toenhance the flow rate through conduit 31. In this design, removal ofdesalinated water 32 through outlet port 33 transverse to the currentdirection near the cation exchange membrane 16 may be enhanced byreduced porosity, increased pore size, or anisotropic porousmicrostructures of the cationic porous medium 12 in the region near themembrane 16 compared with elsewhere in the porous medium 12. This showsthe potential importance of graded or non-uniform microstructures inshock membrane systems.

A system schematic diagram for shock desalination and purification usingelectro-osmotic flow (EO flow) is shown in FIG. 18, though theelectro-osmotic flow can be supplemented with pressure-driven flow. Mostimpurity particles entrained in the incoming sea water or brackish water38 fed through liquid inlet port 40 pass through the channel 50 betweenthe anode 19 and the cationic porous media 12 to a brine output 42ejected through a waste liquid outlet port 44. Additional impurityparticles are filtered by the porous media 12. Cationic ions in theliquid pass from left to right and exit through the cation exchangemembrane 16 into channel 50 to a brine output 46 through outlet port 48.An additional pressure-driven liquid flow is provided through channel 50flush the brine 46 through the outlet port 48.

Another method and system for water recovery driven by transversepressure-driven flow (downward flow of sea water inputs 38, asillustrated, without EO flow) is shown in FIG. 19. The components ofthis system can also be periodically repeated in a parallel stack toboost flow rate. In this embodiment, the shock region 28 anglesdiagonally across the cationic porous medium 12. In alternativeembodiments, the gaps 50 adjacent to the anode 19 can be replaced withadditional cationic porous medium; in particular embodiments, theadditional cationic porous medium that replaces the gap can have adifferent porosity than the rest of the cationic porous medium 12.

Another embodiment of the desalination and purification system includingboth cationic and anionic elements with no gap there between is shown inFIG. 20. Here, sea water 38 is pressure-driven into the system throughinlet port 40; the inlet port 40 includes a filter 52 for screening outlarge particles 54 from entering the system. The sea water flows throughdownward through both the anionic porous medium 13 and the cationicporous medium 12. Smaller particles 56 that pass through the filter 52at the inlet port 40 pass through channels 50 at the perimeter of theapparatus to flush out anions that pass through the (porous) anode 19 onthe left and to flush out cations that pass through the (porous) cathode18 on the right as brine output 46.

In the recovery methods of FIGS. 17-20, there may be additional outletports for co-flowing waste water (ahead of the shock region 28, awayfrom the ion-selective surface), which is separated from the indicatedoutlet port for fresh water (behind the shock, closer to theion-selective surface).

A portable, low-power, small-scale embodiment of the shock desalinationand purification system of FIG. 18 is shown in FIG. 21. In thisembodiment, the cationic porous media is porous glass frit with a poresize of 1 micron. The cation exchange membrane is a NAFION membrane, andthe cathode is porous.

Another method of water recovery, shown in FIG. 22, involvesrepositioning one electrode (e.g., the anode 19) to driveelectro-osmotic flow, which can be enhanced by applied pressure, into aporous material (e.g., a cationic porous medium 12) surrounded by themembrane/electrode assembly (e.g., a cation-exchange membrane 16 and acathode 18). A tubular configuration of this embodiment is shown in FIG.23.

The liquid flows for the system of FIG. 14 are shown in FIG. 24. Atleft, the input stream (e.g., sea water) 38 enters the system; and atright, desalinated and purified (fresh) water 32 is extracted betweenconcentric flows of brine 42 and 46 from the system.

System optimization based on a trade-off between flow rate anddesalination factor is shown in FIGS. 25 and 26. A low flow rate is usedin FIG. 25, wherein the shock 28 extends past the fresh water outlet 33yielding good desalination (i.e., low salt concentration in the “fresh”water 32). At high flow rates, as shown in FIG. 26, the shock staysclose to the cathode 18 and salty inlet water 38 ends up in the “fresh”water output 32 exiting port 33. The slow flow system of FIG. 25 alsohas a power requirement that is higher than that of the fast flow systemof FIG. 26.

System optimization based on a trade-off between water recovery(Q_(fresh)/Q_(in)) and desalination (and power requirement) is shown inFIGS. 27 and 28 by controlling the outlet flow ratio(Q_(fresh)/Q_(brine)), with Q_(in), I, etc., fixed. Low water recovery(Q_(fresh)/Q_(in)) is achieved in FIG. 27, though excellent desalinationis achieved at high energy cost/volume. High water recovery(Q_(fresh)/Q_(in)) is achieved in FIG. 28, though a substantial amountof salt ends up in the fresh water output 32.

Principles of water purification and disinfection by desalination shocksare illustrated in FIG. 29. In this embodiment, the cationic porousmedium 12 provides (ultra) filtration by size. The shock 28 rejectsanionic particles 57 into the brine output 42. Consequently, the freshwater output 32 from the system contains only very small positivelycharged particles, which tend to be rare in water. Alternatively, wherean anionic porous medium is used, cationic particles are rejected intothe brine output, and the fresh water output includes only very smallnegatively charged particles. Moreover, all particles reaching the freshwater outlet experience large electric fields (e.g., >100 V/mm) in thelow conductivity desalinated region behind the shock, thereby providingan effective means of disinfection. Such fields are sufficient to killor neutralize living microorganisms, bacteria, fungi, spores, cells,viruses, etc.

In some embodiments, the outlet stream with higher concentrations ofsalt and co-ionic particles is a desirable product stream in continuouschemical processing, e.g., to produce more concentrated solutions ofacids, electrolytes, colloidal particles, quantum dots, or smallbiological molecules or micro-organisms. In such embodiments, the lowerconcentration stream may be considered as waste.

A schematic illustration of a shock desalination and purification system10 is shown in FIG. 30. The components include a porous medium 12 in theform of silica glass frit with a mean pore size whose order of magnitudeis 1 micron in water (somewhat larger than the maximum Debye length) andan electrode membrane assembly, wherein the membrane 16 is a 2-mm-thinNAFION membrane (NR-212). Fused quartz glasses are used as packagingmaterials. The size of this prototype system is about 1 cm×1 cm×2 cm(cross-section×length). In this embodiment, the liquid (e.g., sea water)38 enters through an aperture in the anode 19 and exits through anaperture in the cathode 18; and a voltage source 62 is coupled with theanode 19 and with the cathode 18 to drive current through the system 10.

In a simple, steady-state, one-dimensional model, an ionic solution isin porous media between an electrode and an ion-selective surface(membrane or electrode). The boundaries are assumed to be porous toallow for analyte flow, where the velocity is assumed to be uniform plugflow. At one end, the concentration of analyte is held constant; and atthe other end, the cation is consumed by the ion-selective surface dueto the applied current. Three dimensionless groups represent the physicsin this model:

${I = {{\frac{i\; L}{{eDC}_{0}}\mspace{14mu}{Pe}} = {{\frac{UL}{D}\mspace{14mu}\gamma} = \frac{\rho_{s}}{e\; C_{0}}}}},$where i is the applied current density, L is the distance betweenelectrodes, D is the diffusivity, C₀ is the original concentration ofanalyte, U is the flow velocity and _(s) is the surface charge pervolume of the pores.

Numerical simulations based on this theory are presented, as follows.The plot in FIG. 31 shows how the conductivity profile, which isproportional to charge concentration, changes as higher voltages areapplied between the two electrodes for a constant Peclet (Pe) number;the chart shows plots for the following voltages: 5.0 volts (plot 64),1.0 volts (plot 66), 0.5 volts (plot 68), and 0.1 volts (plot 70). Apotential is applied at x=1, and the cation species are consumed at arate proportional to the resulting current. After the applied voltagereaches a critical value, a depleted region develops and grows withhigher voltages. The plots in both FIGS. 31 and 32 are for a fixedsurface charge density to initial bulk concentration ratio, γ, set to10⁻². FIG. 32 shows the conductivity profile for Pe values of 0 (plot72), 5 (plot 74), and 10 (plot 76), where voltage remains constant,though flow rate is varied. As the dimensionless velocity, Pe,increases, the depleted region contracts toward the membrane. If theflow rate continues to increase, the depleted region will disappearaltogether.

Classically, when super-limiting currents are applied, the resultingvoltage will increase towards infinity as the system tries to satisfythe applied boundary condition. However, based on this theory, under theright conditions, the applied voltage will level off to a stable,steady-state value, as seen in FIG. 33. This figure includes V-I curvesfor varying surface charge densities ( ) and Pe numbers. Specifically,surface charge densities (and Peclet numbers are plotted as follows:

-   -   =0.0001 and Pe=0 (plot 78), =0.0001 and Pe=5 (plot 80), and        =0.0001 and Pe=10 (plot 82),    -   =0.001 and Pe=0 (plot 84), =0.001 and Pe=5 (plot 86), and =0.001        and Pe=10 (plot 88),    -   =0.01 and Pe=0 (plot 90), =0.01 and Pe=5 (plot 92), and =0.01        and Pe=10 (plot 94),    -   =0.1 and Pe=0 (plot 96), =0.1 and Pe=5 (plot 98), and =0.1 and        Pe=10 (plot 100).        The jump in voltage corresponds to a depletion region        developing. As Pe increases, the overall behavior remains the        same; but the location of the “jump” (i.e., a voltage plateau)        occurs at higher currents. As surface charge, decreases, the        system approaches the classic case of diffusion-limited current,        where the voltage goes to infinity.

With the development of steady-state depletion regions, where thedepleted region represents desalinated water, this method may be usedfor desalination. To evaluate the viability of this method for waterdesalination, the energy/volume of a sample system was considered. Asimple one-dimensional model was used to predict the energy efficiency(contour lines) of purification via a shock membrane for seawaterdesalination for different applied voltages at different flow rates, orPeclet numbers, UL/D. In FIG. 34, energy/volume is plotted under varyingconditions for 0.5M saltwater. The theoretical energy/volume limit is 1kWh/m³ and is represented by the left-most line 102. Line 104 is thecontour of 4 kWhr/m³, which is the best case scenario for reverseosmosis. The straight, angled lines are contours of the degree ofdepletion at the end of the channel, where the lines represent from 1%depleted (line 106), 0.1% depleted (line 108), and 0.01% depleted (line110). In order to achieve higher depletion, higher voltages or lowervelocities can be employed. These lines, in conjunction with thephysical limit of about 1V (in water), demonstrate some of the designconstraints for this problem. While this method could compete withreverse osmosis at low flow rates, it may also be beneficial to create asmall, portable system with higher water production for a slightlyhigher energy cost.

The larger the surface charge ( ) on the porous material is, thestronger the nonlinear force will be, resulting in higher currents atlower voltages. In FIG. 33, the voltage vs. current curves increasesharply as the system approaches depletion (around I=1 for Pe=0), andthen the slope decreases corresponding to the depleted region. The valueof the voltages in the depleted region can change over orders ofmagnitude depending on the surface charge. As a result, it isadvantageous to use a porous material with a high surface charge.

Additionally, under optimal conditions, a desalination system would notbe run with large depletion regions. It is more efficient (see FIG. 34)to operate under the lowest voltages possible. Therefore, the system canbe optimized so that the depletion region is as small as possible whilestill maintaining good water recovery by balancing flow rate and appliedvoltage. Increasing the flow rate improves efficiency and decreases thedepleted region (see FIGS. 31 and 32), making water recovery difficult.Increasing the applied voltage increases the depletion region butdecreases the efficiency. Consequently, optimization of water productionand efficiency may depend on these factors.

Though water desalination and purification have been specificallydiscussed, the systems and methods can likewise be used with otherliquids in other contexts, such as for the segregation ofsurfactant-stabilized colloids or inverse micelles in a non-polarsolvent, where an electrode is substituted for the membrane. Forexample, electrophoretic displays, such as the displays produced byE-Ink (Cambridge, Mass.) for the Amazon Kindle electronic reader includeblack and white, oppositely charged colloidal particles suspended in aliquid near a transparent electrode. When a voltage is applied, black orwhite particles crowd on the electrodes and change the color of the“pixel.” The pixel is a set of oil droplets squashed between parallelplate electrodes. Accordingly, the electrolyte in this embodiment is anon-aqueous, non-polar solvent. The charged particles can be eithersurfactant-stabilized colloids, which would not dissolve in oil withoutthe surfactant molecules, or inverse micelles (i.e., clumps ofsurfactants). In this context, the system can be used to reduce/controlthe concentration of charged particles in a controlled way, in alarge-scale continuous process, e.g., in the production of electronicinks.

In another embodiment, the methods and apparatus of the invention can beused to continuously produce an outlet stream of increased concentrationof dissolved salts or small, charged impurities, such as nanoparticles,quantum dots, colloidal particles, organic molecules, minerals,biological molecules, small proteins, DNA, microorganisms, cells, andviruses. The higher concentration of salt or impurities can be used toenhance the sensitivity (signal to noise ratio) of detection methodsdownstream of the device. The same method and apparatus can also be usedto continuously increase the concentration of charged particles inchemical mixtures, colloids, electrolytes, acids, etc., in applicationssuch as water softening, food processing, and chemical production.

In another embodiment, the methods and apparatus of the invention can beused in conjunction with an electrodeposition/dissolution cell, wherethe ion-selective surfaces are electrodes rather than ion-exchangemembranes. For example, the electrolyte can be an aqueous solution ofcupric chloride (CuCl₂), and the cation-exchange membrane can bereplaced by a metallic copper cathode, polarized by an applied voltageto deposit copper from solution (Cu⁺²+2 e⁻→Cu). The anode can also bemade of copper, and the reverse reaction (dissolution) will occur inresponse to the voltage to produce cupric ions. In this situation, acationic porous medium placed in contact with the cathode will lead tothe formation of desalination shocks and allow the passage ofover-limiting (or super-limiting) current. This system will also depletethe salt concentration and remove impurities from the region between theshock and the electrode. The same procedure can be applied to anyelectrochemical cell, wherein electrodes act as ion-selective surfacesdepleting the local salt concentration.

In other embodiments, the methods and apparatus can be used fordisinfection. The electric fields can be very large near the shockregion 28 where water is extracted. Most biological impurities havenegative charge in water and will be rejected by charge and by size froma cationic porous medium 12. Accordingly, biological organisms can beremoved from the liquid stream using this apparatus and methods.Likewise, other contaminants, such as heavy ions, can be removed from aliquid stream using the apparatus and methods.

Experimental:

Our basic strategy to extract pure water behind desalination shocks isillustrated in FIGS. 30 and 35-37, along with the first experimentalprototype. The experimental setup, as shown in the schematicillustration of FIG. 30, includes a sandwich structure of a 1-mm-thick,1-cm-radius porous silica glass frit 12 (commercially available withsubmicron pores—in this case, with pore widths ranging from 50 nm to 1micron) against a NAFION membrane 16, supported by a plastic mesh withsolution reservoirs 50 leading to metal electrodes on either side. Thefrit/membrane assembly is packed in a hard plastic with screws to formtight seals at the outer edges. Fresh water 32 is continuously extractedfrom the depleted zone 30 within the glass frit 12, which passesover-limiting current through the NAFION membrane 16. Brine 42 and 46 isproduced in the anode and cathode compartments 50 and removed by a slowpressure-driven flow. A cross-sectional CAD drawing of the structure isprovided in FIGS. 35 and 36.

For continuous water extraction, a ˜100-micron-thick circular orifice onthe frit side wall up to the membrane interface leads to an O-ringchannel where fresh water collects before it proceeds through an outletvalve 33. The outlet flow 32 can be precisely controlled by a syringepump with velocity precision down to microns/sec, although eventuallythe device may operate using spontaneously generated electro-osmoticflows in the glass frit 12, without needing any externally appliedpressure. Solution conductivities in different locations are measured byimpedance spectroscopy, either by extracting a sample into a capillarywith electrode caps, or by making electrical measurements with in situelectrodes. pH levels will also be monitored.

As a model system with simple chemistry, a prototype apparatus, shown inFIG. 37, was used in first experiments involving aqueous copper sulfatesolutions with copper electrodes 18 and 19 undergoingdeposition/dissolution reactions. In that case, the copper cathode 18 isin direct contact with the membrane/frit assembly, and we find thatuniform deposition occurs without forming dendrites up to several volts.The anode 19 is a copper ring on the other side of the anodic reservoirfeeding the glass frit from the back. In a more general setup, theelectrodes are separated from the reservoirs and catalyze waterelectrolysis reactions away from the membrane, as in electrodialysissystems.

The preliminary copper sulfate results in FIGS. 38-40 are promising andshow good agreement with our theoretical predictions. The observedcurrent-voltage relation in steady state with zero net fluid flow fitour analytical formula, thus supporting the hypothesis of over-limitingcurrent carried by surface conduction in our submicron channels. This isalready an important result, since we have demonstrated over-limitingcurrent by a new mechanism in a nano-porous medium where convection issuppressed.

Our first proof-of-concept desalination experiment to extract water fromcopper sulfate solution in the frit was also successful, using only 1.5Volts. A simple measure of the energy efficiency from the data yieldsthe following: energy/volume=power/flow rate=(2 mA*1.5V)/(2 μL/min)=23kWh/m³, which is satisfactory considering that the process had not yetbeen optimized.

The data from our first attempt at continuous desalination of “brackishcopper sulfate” (100 mM) is shown in FIG. 38, showing the conductivitydecrease by up to a factor of 20 at low flow rates (down to “potable”levels) in a device with a 500 micron gap on a 1 mm glass frit. Using asmaller gap should dramatically improve the energy efficiency andconcentration reduction. FIGS. 39 and 40 plot the impedance spectra forthe input and output solutions, respectively via copper probe electrodesshowing the increase in bulk resistance before and after passing throughthe device, indicating strong depletion of the mobile ions.

In this first device, the outlet gap of 500 microns (half of the fritthickness of 1 mm) was overly wide, thereby allowing much of theconcentrated diffusion layer to exit the frit along with the depletedzone. As a result, the salt (copper sulfate) concentration was onlyreduced by a factor of 20 from brackish levels (100 mM) to potablelevels (<10 mM), but we expect better results from a planned device witha 100 micron gap. The theory predicts that the depleted zone will reacha salt concentration comparable to the number of surface charges pervolume in the porous medium, which is <0.1mM.

A simple conservation analysis for thin desalination shocks gives anultimate efficiency of E/V=P/Q≈t_c₀eV, where t_is the co-iontransference number, c₀ the salt concentration, and V the appliedvoltage. For small applied voltages, near the thermal voltage V=25 mV,the predicted energy density of shock-membrane purification approachesthe thermodynamic lower bound, set by the osmotic pressure (0.7 kWh/m³for seawater). In this limit, however, the shock width becomescomparable to the depletion zone width, which interferes with therecovery of the fresh water, due to excessive mixing of the fresh andsalty regions, which makes sense, since we cannot beat thermodynamics.Careful engineering of this system, however, will help to optimize thetrade-off between efficiency, flow rate, and water recovery, and reachuseful performance metrics for applications.

In describing embodiments of the invention, specific terminology is usedfor the sake of clarity. For the purpose of description, specific termsare intended to at least include technical and functional equivalentsthat operate in a similar manner to accomplish a similar result.Additionally, in some instances where a particular embodiment of theinvention includes a plurality of system elements or method steps, thoseelements or steps may be replaced with a single element or step;likewise, a single element or step may be replaced with a plurality ofelements or steps that serve the same purpose. Further, where parametersfor various properties are specified herein for embodiments of theinvention, those parameters can be adjusted up or down by 1/100^(th),1/50^(th), 1/20^(th), 1/10^(th), ⅕^(th), ⅓^(rd), ½, ¾^(th), etc. (or upby a factor of 2, 5, 10, etc.), or by rounded-off approximationsthereof, unless otherwise specified. Moreover, while this invention hasbeen shown and described with references to particular embodimentsthereof, those skilled in the art will understand that varioussubstitutions and alterations in form and details may be made thereinwithout departing from the scope of the invention. Further still, otheraspects, functions and advantages are also within the scope of theinvention; and all embodiments of the invention need not necessarilyachieve all of the advantages or possess all of the characteristicsdescribed above. Additionally, steps, elements and features discussedherein in connection with one embodiment can likewise be used inconjunction with other embodiments. The contents of references,including reference texts, journal articles, patents, patentapplications, etc., cited throughout the text are hereby incorporated byreference in their entirety; and appropriate components, steps, andcharacterizations from these references optionally may or may not beincluded in embodiments of this invention. Still further, the componentsand steps identified in the Background section are integral to thisdisclosure and can be used in conjunction with or substituted forcomponents and steps described elsewhere in the disclosure within thescope of the invention. In method claims, where stages are recited in aparticular order—with or without sequenced prefacing characters addedfor ease of reference—the stages are not to be interpreted as beingtemporally limited to the order in which they are recited unlessotherwise specified or implied by the terms and phrasing.

What is claimed is:
 1. A desalination and purification system for asource liquid containing co-ions and counter-ions selected fromdissolved ions, charged impurities, charged droplets and combinationsthereof, wherein the co-ions and counter-ions have charges of oppositesigns, the system comprising: a conduit for liquid flow including aliquid inlet port, a desalinated/purified liquid outlet port, and awaste liquid outlet port; a first electrode; a second electrode andion-selective boundary configuration selected from: a) a secondelectrode and at least one distinct ion-selective boundary, and b) asecond electrode that also serves as the ion-selective boundary, whereinthe electrodes are configured to drive ionic current in the sourceliquid when the source liquid fills the conduit and to producebrine/waste liquid from the source liquid, and wherein the ion-selectiveboundary is configured to conduct the ionic current and selectivelytransmit or remove the counter-ions and block the co-ions from thesource liquid; and at least one porous medium, wherein in configuration“a” the porous medium is on an opposite side of the ion-selectiveboundary from the second electrode, wherein the porous medium has asurface charge with a sign that is the same as the sign of the co-ionsto enable conduction of an additional ionic current via the counter-ionsand consequent production of a region of desalinated/purified liquid,wherein the desalinated/purified liquid outlet port is positioned toextract the desalinated/purified liquid from the porous medium, whereinno membrane is positioned in the porous medium to create a separation ofliquid flow paths from the liquid inlet port between (i) thedesalinated/purified liquid outlet port and (ii) the waste liquid outletport.
 2. The desalination and purification system of claim 1, whereinthe source liquid is coupled in fluid communication with the liquidinlet port and comprises water that contains dissolved salts and atleast one of the following: suspended particles and suspended droplets.3. The desalination and purification system of claim 2, wherein thedesalinated/purified liquid outlet port contains desalinated/purifiedliquid, and wherein the waste liquid outlet port contains waste liquid.4. The desalination and purification system of claim 1, wherein only oneion-selective boundary is included in the system, and wherein theion-selective boundary separates only one electrode from the liquidinlet port.
 5. The desalination and purification system of claim 1,wherein channels defined by the porous medium have a mean diameter of atleast about 10 nm.
 6. The desalination and purification system of claim1, wherein the porous medium is selected from porous glass, porousceramic, porous metal oxide, porous polymer, porous functionalizedpolymer, porous cross-linked polymer, and porous zeolite materials. 7.The desalination and purification system of claim 1, where the porousmedium is an array of channels fabricated in silica glass, silicon, or apolymeric material.
 8. The desalination and purification system of claim1, wherein the porous medium is a packed bed of particles havingdiameters of about 100 microns or less.
 9. The desalination andpurification system of claim 1, wherein the porous medium has at leastone of (a) non-uniform porosity, (b) non-uniform surface charge, and (c)non-uniform microstructure when viewed across a distance at least anorder of magnitude greater than an average diameter of the pore channelsin the porous medium to generate non-uniform liquid flow or control ofthe salt concentration profile across different regions of the system.10. The desalination and purification system of claim 1, where one orboth electrodes are also porous.
 11. The desalination and purificationsystem of claim 1, wherein the ion-selective boundary and porous mediumare selected from at least one of the following: (a) a cation exchangemembrane positioned between the second electrode and the porous medium,wherein the second electrode is a cathode, and wherein the porous mediumhas a negative surface charge and (b) an anion exchange membranepositioned between the second electrode and the porous medium, whereinthe second electrode is an anode, and wherein the porous medium has apositive surface charge.
 12. The desalination and purification system ofclaim 11, wherein the membrane and the porous medium comprise a firstlayer in an alternating sequence of at least two stacked layers ofmembranes and porous media with internal charge of the same sign,terminating with another ion-selective boundary of the same internalcharge sign as the internal charge of the membranes and porous media,placed in between the two electrodes, wherein each layer includes arespective liquid inlet port, desalinated/purified liquid outlet portand waste liquid outlet port, and wherein no membrane is positioned inthe porous medium in any of the layers to create a separation of liquidflow paths from the liquid inlet port between (i) thedesalinated/purified liquid outlet port and (ii) the waste liquid outletport.
 13. The desalination and purification system of claim 11, whereinthe second electrode is porous and the ion-exchange membrane isconnected to the second electrode in a membrane-electrode assembly. 14.The desalination and purification system of claim 1, wherein theion-selective surface is also the second electrode and selectivelyremoves counter-ions either by capacitive charging of the double layersor by electrochemical reactions.
 15. The desalination and purificationsystem of claim 1, wherein the porous medium has non-uniform pores,including at least one of the following: graded pore structure,discontinuous changes in pore structure, or pores ranging in size by afactor of at least five.
 16. The desalination and purification system ofclaim 1, wherein the porous medium is an agglomerate of different porousmaterials.